The invention relates to a display device comprising a display panel with a first substrate provided with electrodes at the location of pixels, a second light-transmitting substrate and liquid-crystalline material between the two substrates, an illumination system which is situated on the side of the second substrate facing away from the liquid-crystalline material and which includes a light guide of an optically transparent material having an exit surface facing the display panel and a number of end faces, an entrance face for light being situated opposite at least one of said end faces, whereby the light can be coupled in at that end face of the light guide.
Such reflective display devices are used, for example, in portable equipment such as laptop computers, mobile telephones, personal organizers etc. With a view to saving energy, it is desirable that the light source can be turned off when there is sufficient ambient light.
A display device of the above-mentioned type is described in "Compact Front Lighting for Reflective Displays", SID 96 Applications Digest, pp. 43-46. In said document a description is given of a light guide to which an optical compensator is added to preclude image distortion due to a groove structure (microprisms) on the viewer's side of the light guide. Image distortion can be attributed to the fact that the groove structure has different slopes, which results in multiple images. The groove structure is necessary to deflect light rays in the direction of the display panel. In said publication, multiple images are precluded by providing an optical compensator having a complementary groove structure.
In order to function properly, the air gap between both component parts (the light guide and the optical compensator) must have a small accurately defined thickness (obtained, for example, by means of spacers) throughout the plane. This requires a very accurate mounting step, while each local variation in thickness of the air gap gives rise to variation in optical behavior and image distortion.